In calcination systems, materials are heated to high temperatures without fusing the materials but rather to effect useful chemical changes such as oxidation or reduction. Calcination is also used to expel the volatile portions of a substance by heat. Various heating processes or systems are known to affect calcination of materials. Thermal processing of a material generally requires gas-solid contact to dry the material and sometimes to free the material of all residual acids at temperatures below the melting point of the material. This is accomplished by the use of drying equipment such as commercial dryers or calcination kiln systems. Some of the prior art dryers used are gravity dryers, roto-louvre dryers, heat vibrating conveyor dryers, rotary kilns, direct and indirect heat rotary systems and stem tube dryers.
This invention is concerned with and intended primarily for both calcination systems which both dry a material and cause a chemical reaction or chemical change in the material being processed. However, if a particulate material is to be only dried, this present system can be used. By "batch" as used throughout this disclosure is meant a system, process or apparatus used for or in batch as opposed to a continuous process. There are known two types of direct-heat rotary calcinating equipment, those using longitudinal cocurrent gas-solid flow and those using longitudinal countercurrent flow of gas and solids. Usually, somewhat greater heat-transfer efficiency is obtained by using countercurrent flow; cocurrent flow can be used to process heat-sensitive materials at higher inlet gas temperatures. In both of these rotary calcination systems, better heat-transfer efficiency and better material yield are somewhat difficult to achieve with prior art equipment. An additional concern using both cocurrent flow and countercurrent flow is the possibility of substantial attrition of the material being processed. Several prior art calcination systems are disclosed in Perry's Chemical Engineers' Handbook, pages 20-30 to 20-75, Sixth Edition, McGraw-Hill Book Company.
Thus, calcination is distinguished from dryers in that calcination not only dries a material by heat but also effects a chemical change in the material such as reducing mineral high oxides to low oxides, calcination of silica gel, removal of sulfur from cobalt, copper and nickel, reduction of metal oxides, oxidizing and burning off of organic impurities, reduce a metal oxide to a metal and other various chemical changes obtainable in calcination systems. The present invention is related to calcination systems and not only pure drying systems. Thus, the calcining system of the present invention is designed to operate at elevated temperatures of at least below the melting point of the particulate material treated which is substantially higher than most heat dryers.
As earlier noted, in rotary calcination systems, the heated gas is introduced and passed through the system in either cocurrent or countercurrent gas flow fashion. The present invention uses batch rotary calcination equipment that provides gas cross-flow through the material being treated. The energy utilization is optimized in the present process as compared with the prior art processes and hold-up of material is substantially eliminated.
Some of the related prior art processes arc disclosed in U.S. Pat. Nos. 1,185,899; 3,645,010; 4,535,550; 5,083,870; 5,052,809 and 5,312,599.
In U.S. Pat. No. 1,185,899 (Geiger), a hot air dryer is disclosed wherein heated air is provided through pipes that rotate inside the rotating drum. Air is introduced into Geiger's system, it then travels longitudinally through tubes 16, through the cylinders 13 and 17 and ultimately out through air outlet 34. In order to produce sufficient draft through the dryer and to ensure that the draft will be properly directed, a fan or blower 32 is employed. In operation, the cylinders 13 and 17 are rotated and the material to be dried is fed into the cylinder 17 through the hopper 25 from which it advances into the cylinder 13 by the continued rotation of the cylinders. The blower used by Geiger is disposed at either end of the dryer as shown in his drawing. When the blower is positioned at the discharge end of the dryer, it is necessary to provide a discharge stack 45 at the other end of the dryer.
Korekawa U.S. Pat. No. 3,645,010 discloses a clothes dryer in which wet clothes introduced into a rotating drum are stirred and dried. The heated air is introduced into Korekawa's dryer via small holes 13 and small holes 25 provided on protruded portions 21. The warm air introduced into the drum 10 is discharged out of the exhaust port 8 through the duct 9 by way of a filter 16. The rotating drum of Korekawa is provided with a projection on the inner surface of the drum for stirring the clothes put therein. Korekawa's structure is quite different from a calciner where much more elevated temperatures are used.
The Walter U.S. Pat. No. 4,535,550 discloses a drying apparatus 12 that includes a gas a gas supply pipe 34 that extends axially through annular plates which are used to retain the tumbling bed 38 of solid particles being dried. A number of radially extending pipes 40 pass from the supply pipe 34 and supply the processing gas to conduits 42 for distribution within the tumbling bed 38 of the material being treated. In drying particulate material by the evaporation of water, the drying gas can be held at temperatures of up to 800.degree. C. At least one air conduit 42 which is in communication with supply pipe 34 extends longitudinally within the vessel 12.
In Sindelar U.S. Pat. No. 5,083,870, an apparatus for manufacturing an asphalt paving composition is disclosed. The portable asphalt machine of Sindelar has a rotating drum with heat zones dispersed along the unit. Gas exits this prior art structure via exit ports 115 and is exhausted through scrubber exhaust 163.
The rotary asphalt dryer of Young U.S. Pat. No. 5,052,809 has a drum having an axis of rotation and a mechanism for introducing materials therein. The gases used in Young's device flow from right to left (as shown in FIGS. 1 and 2) under the influence of a blower and as secondary air is drawn into the inner shell through a secondary air supply means.
A rotary furnace for making chemically active carbon is disclosed in U.S. Pat. No. 5,312,599 (Schwartz). In the Schwartz apparatus, reaction gases are introduced into the system by a tube 26 which has a plurality of outlet ports 42. Positioned also within the furnace 20 are gas exhaust tubes 30 which are disposed in parallel spaced relationship to the inlet tubes 26.